Sumped manholes are commonly used in sewer systems to temporarily collect settleable solids until they can be removed from the system during routine maintenance. As illustrated in FIGS. 1 and 2, a typical sumped manhole 12 includes a cylindrical manhole sidewall 14 and a bottom 16. An inlet pipe 18 extends into manhole sidewall 14 via an inlet hole 20 in manhole sidewall 14, and an outlet pipe 22 extends into manhole sidewall 14 via an outlet hole 24 in manhole sidewall 14. Both inlet pipe 18 and outlet pipe 22 are spaced above bottom 16 of sumped manhole 12 to form a sump 26 below inlet and outlet pipes 18 and 22 above bottom 16 of sumped manhole 12.
In a drain system 10 including a typical sumped manhole 12, fluids flow into sumped manhole 12 via inlet pipe 18 and out of sumped manhole 12 via outlet pipe 22, as generally indicated with arrows 28. The fluids moving from inlet pipe 18 to outlet pipe 22 carry solids, such as sediment and larger waste items, and drop at least a portion of the solids carried therewith into sump 26. The solids collected in sump 26 include sediment 32, which collects on bottom 16 of sumped manhole 12 for subsequent removal during periods with a low flow rate. During periods of high flow rates, the high energy flow jet substantially linearly extends between inlet pipe 18 and outlet pipe 22 introducing a circular flow pattern, as generally indicated by arrow 30 in FIG. 2, below the primary flow 28. Circular flow pattern 30 interrupts collected sediment 32 in sump 26 resulting in scour of the collected sediment 32 in sumped manhole 12. Scour is the undesirable process of high fluid flows transferring sufficient energy to previously settled sediment 32 in a manner re-suspending sediment 32 in the water column and subsequently washing such sediment 32 out of sumped manhole 12 and downstream. In one observed experiment and as illustrated in FIG. 2, due at least in part to circular flow pattern 30, sediment scoured from a downstream portion of sump 26 to a more upstream portion of sump 26 relative to inlet pipe 18, resulting in a higher total sediment height in sump 26 as compared to an initial solids level, which is generally indicated with a dashed line at 34.
Until recently, the effectiveness of standard sumped manholes 12 at removing settleable solids has not been quantified, and was assumed to be marginal. Due to the assumed marginal removal efficiencies of standard sumped manholes 12, several products have been developed that claim to greatly improve the performance of sumped manholes 12 via the addition of internal components to sumped manhole 12. These products focus on designs that claim to increase removal efficiencies, reduce scour, or both.
One such product is a floatables skimmer 40 as illustrated, for example, in FIGS. 3 and 4 as added to sumped manhole 12 as originally presented in FIGS. 1 and 2. Skimmer 40 is formed of a substantially solid material restricting fluid flow therethrough and is coupled to manhole sidewall 14 on either side of outlet pipe 22. In this manner, skimmer 40 extends both above and below a top and a bottom of outlet pipe 22, respectively. While skimmer 40 serves to decrease floating larger solids and smaller solids alike from rushing with the fluid flow 28 into outlet pipe 22, skimmer 40 also introduces disruptions to fluid flow within sumped manhole 12. As illustrated in FIGS. 3 and 4, for example, the circular arrows 72 generally indicate vortex flow patterns 42, which scour sediment 32 on each opposing side of outlet pipe 22. The scour re-suspends sediment 32 that was previously collected in those areas as shown by the original sediment line 34 in FIG. 4. In recent years, several publicly funded studies have been performed to determine the effectiveness of standard sumped manholes with and without flow treatment products using standardized removal efficiency and scour testing. The results of these studies show that the existing product-modified systems can provide increased removal efficiencies over standard sumped manholes, but can also fall short of desired sumped manhole removal efficiencies.
Furthermore, some systems use internal components to swirl water, which increases particle travel paths, consequently resulting in increased removal efficiencies. While swirling low flows have proven to increase removal efficiencies, swirling flows also have the effect of creating vortices during high flows, greatly increasing scour in comparison to standard sumped manholes. Scour testing has revealed that scour in standard sumped manholes in product-modified systems is a more important factor than removal efficiency in determining a treatment devices typical annualized removal efficiency. Essentially, the removal efficiency of a structure is negated if it is not designed to retain previously settled solids during high flows.
Other storm water treatment systems configured to improve the performance of standard sumped manholes by focusing on scour suppression, such systems often utilize a horizontal false floor. While a false floor is relatively effective at suppressing scour by providing a boundary between previously settled solids and high flows, the false floors introduce negative side effects such as reducing sediment removal efficiencies by effectively reducing the water depth and creating an obstruction for routine inspection and maintenance. Use of false floors is generally restricted for use within circular manholes and such false floors are not retrofittable, thereby limiting the overall applicability of such false floors.
In view of the above-described issues with existing storm water systems, there is room for improvement of standard sumped manholes and modifying products currently on the market.